Steve Zaillian’s adaptation of Robert Penn Warren’s 1946 Pulitzer Prize–winning novel All the King’s Men, about the making of a demagogue—modeled on Louisiana governor (and later senator) Huey Long—is languid, undramatic and shapeless. Zaillian has a talent for streamlining big, incident-filled books. He wrote the screenplay for Schindler’s List and both wrote and directed A Civil Action. But in this case his love for the novel seems to have done him in.

Warren’s book is lyrical and impressionistic, and despite its size (nearly 600 pages) it relies on the reader’s imagination to fill in the arc of Willie Stark’s rise to power. Stark begins as a decent politician who captures the poor and working class by voicing their outrage. He then becomes a master manipulator, practically a mobster, who barely escapes impeachment. In Zaillian’s movie, Stark (Sean Penn) is first one and then the other.

Stark’s political education is dramatized clearly enough. His first gubernatorial campaign turns out to be a ploy, engineered by the good-old-boy incumbent to split the opposition vote. Willie is too naive to see that he was never intended to win. When he finds out the truth, you can see his face practically break apart with the force of the revelation. (This is Penn’s best scene.)

From that point his stump speeches take on a tone of ironic self-deprecation: he knows that everyone expects him to be dumb, and he realizes that he has been dumb—dumb enough to let himself get stepped over. His new rhetoric excites his constituency. He loses the election, but in the next go-round he mounts his own campaign and wins by a landslide. So we see how Willie loses his innocence. What Zaillian fails to show us is how he loses his principles.

Penn is effective in the quiet, contemplative scenes, but in the big set pieces in which Willie is shown captivating voters, he’s studied rather than convincing. (It doesn’t help him that Willie’s grandstanding calls for the kind of large-scale staging that Zaillian doesn’t know how to pull off.) The truth is Penn is wrongly cast. He can certainly be a powerhouse on screen and can portray a charismatic leader (Casualties of War), but he may be too complex an actor to play an iconic public figure. His presence is too dark, too troubled.

The casting emphasizes exactly the part of Stark’s character that is the least plausible: Stark as the cynical philosopher who knows everyone has a dark, buried secret and who sets reporter Jack Burden (Jude Law) to the task of digging up dirt on Judge Irwin (Anthony Hopkins), Jack’s godfather and Willie’s most respected and outspoken detractor.

Penn has an impressive cast behind him, but none of them gives a distinguished performance. Kate Winslet plays Anne Stanton, the love of Jack’s life. Mark Ruffalo (a worse case of miscasting than that of Penn) is her idealistic brother Adam, Jack’s closest childhood friend, now the brilliant doctor whom Willie wants to run the hospital that he is planning to build as tribute to himself. Kathy Baker is uncharacteristically terrible as Jack’s mother, a Tennessee Williams–style fading-southern-belle type. James Gandolfini is Tiny, a political enemy whom Willie first embarrasses and then hires. Patricia Clarkson plays Sadie Burke, a crony of Stark’s who becomes his lover.

Clarkson has the right brittle quality for Sadie, and she might have given a memorable performance if Zaillian had written her a scene or two that sketched in her relationship with Willie, so we wouldn’t have to take their romance on the strength of hearsay. The best sustained work is by Jackie Earle Haley in the minor role of Willie’s loyal driver and bodyguard.

Burden is in some ways like the Nick Carraway character in The Great Gatsby; he narrates the story from the sidelines. But Warren made Burden the emotional center: the novel emphasizes Burden’s fall from innocence, not Willie’s. In dramatic terms that’s a mistake. Burden, the hard-drinking romantic who lets everything he cares about in his life slip away and who waxes poetic about it in voice-over, is a writer’s conceit, like the figure of Carraway. As moviegoers, we want the focus to be on Stark.

As Burden, Law gives a droning, colorless performance. But it’s hard to think of an actor you might want to see in the part. The first movie version of the book, which won an Oscar in 1949, eliminated the poetry, pared down the plot and turned the story into a melodrama with a figure of frightening strength at its center. It was gaudy, electric entertainment. Zaillian resists melodrama with every fiber of his being, but going with melodrama may be the only way the material can be played on the screen.